Strain-Induced Nematic Phase Separation in Polymer Melts and Gels

Recent experiments strongly suggest the existence of nematic-like short-range orientational interactions in polymer melts and networks. In this paper we study a mean-field model which includes a Maier-Saupe interaction between monomers of polymer chains in a melt or a polymer network and find that external strain can induce phase separation in a mixture of chemically identical species which differ only in molecular weight. This mechanism for phase separation results from an exchange of mixing entropy, lost upon separating the small chains from the longer chains, for nematic free energy, gained by orienting the longer chains. The essential point is that the external strain field acts directly only on the longer chains. A detailed analysis of the spinodal condition, which incorporates elastic strain and orientational degrees of freedom, reveals that the interplay between strain and orientational fluctuations induces anisotropic spinodal decomposition in a blend under uniaxial strain: the peak in the accompanying structure factor lies either along or perpendicular to the axis of strain, depending on material parameters.